The present invention relates generally to printing technologies and more particularly to automatic detection of black and white pages in a color document stream for printing.
Conventional printing systems often include a digital front end (DFE) or a printer server. The DFE or print server is a computer system that performs some processing on print jobs and distributes the processed print jobs to one or more printers for printing. Often times, a print job contains a document that is encoded in a page description language, such as PCL or Postscript. The DFE or print server has an interpreter for interpreting page description language commands contained in the document to produce color data for the pixels representing the image of each page. This color data may be passed to a printer to produce a printed version of the page. If the document is a color document, the interpreter produces four color separations for each page: a cyan color separation, a magenta color separation, a yellow color separation and a black color separation. Each color separation is matched with an ink color and contains color data for each pixel specifying how much of the color associated with the color separation is to be used for coloring the pixel. The ink colors may be combined to produce a roughly infinite variety of colors. The ink colors have a subtractive effect relative to each other, where each layer of ink removes a color from light reflected by the printing substrate. For example, yellow removes the blue light components reflected from a white paper substrate. Each color separation holds color data for each pixel on the page. The data may identify what amount of the color associated with the color separation is to be combined with the other colors associated with the color separations to produce a desired color.
With conventional printing systems, the interpreter does not report when a black and white page (i.e. a monochromatic page) is found within a color document. As a result, all of the pages are treated as color pages during printing. Color pages typically require four printing passes, where each printing pass is associated with a specified color (e.g., cyan, magenta, yellow or black). In contrast, a black and white page requires only a single printing pass. Hence, where a black and white page appears in a color document, the black and white page is printing with four printing passes rather than one printing pass. This conventional approach is both time consuming and wasteful of storage space.
The present invention addresses the above-described limitation of conventional printing systems by providing a device, such as a DFE or a print server, that is able to recognize black and white pages in a color document. The device designates the black and white pages in a color document so that the designated pages may be printed in a black and white mode rather than in a color mode. The color pages are still printed in a color mode. Thus, the document is printed more efficiently without wasting printing resources.
In one embodiment, the present invention identifies whether a page is a black and white page by examining data within the color separations for the page. In particular, predetermined bytes within one of the cyan, magenta or yellow separations may be examined to determine if the bytes have zero values, indicating that there are no cyan, magenta or yellow components for the specified pixels. A black and white document has no color components other than a black color component. This embodiment also examines the respective sizes of the cyan, magenta and yellow color separations after linear compression to ensure that all of these compressed color separations are the same size. It would be expected, given a linear and lossless compression method, that a black and white document would produce compressed color separations of the same size, except for the black color separation. Lastly, this embodiment compares the size of the black color separation to the size of one of the other color separations. If the black color separation is larger and the other tests have been met, the page is determined to be a black and white page.
In accordance with one aspect of the present invention, a device is provided for processing a document having multiple pages with at least one black and white page and at least one full color page. The device includes an interface with at least one image output terminal to which the document may be sent to output the document. The device also includes an analyzer for analyzing each page of the document to determine whether the page is to be output in a black and white mode that is used for outputting monochromatic pages, or in a full color mode that is used for outputting full color pages. The device may be, for example, a digital front end or a print server.
In accordance with another aspect of the present invention, a method is practiced in an apparatus for processing documents prior to passing the documents to at least one image output terminal (IOT). The method processes a selected page of a mixed document that has at least one full color page and at least one black and white page. In accordance with this method, the color data in at least one of the color separations for the selected page is analyzed and a determination is made whether the selected page is a black and white page. Where the selected page is determined to be a black and white page, the selected page is designated for printing in a black and white mode. Where the selected page is determined not to be a black and white page, the selected page is designated to be printed in the full color mode. The method may include the step of decomposing the selected page into color separations. Moreover, the method may include the step of examining a selected portion of the color data in one of the separations to determine whether all the color data in the selected portion has the same value. The color separations may be compressed, and the sizes of the compressed color separations may be used to determine whether the selected page is a black and white page or a full color page.